The present invention relates to a method of purging of waste gases. More particularly, the invention relates to a process for the removal by absorption of noxious components from waste gases, particularly gases generated by the combustion of industrial and domestic wastes.
Methods of separation of noxious gases, such as SO.sub.2 and HCL, from waste gases with wet residues have been known in the art for a long time. These conventional methods have two disadvantages. The first disadvantage resides in that reaction products in the form of aqueous solutions on suspensions load waste water. The second disadvantage is that waste gases are cooled off below their dew point, so that an energy-consuming repeated heating of the gases is required.
It has been suggested to avoid the above disadvantages by a so-called "wet-dry" process, which is described in the German patent publication DE-AS 21 50 186. A reagent solution is spray-injected into the gas stream contained in the reaction container or tower according to the disclosed method. The supply of the reagent solution in the form of the fine mist is determined in dependence upon the concentration of pollutants and the temperature in the purified waste gases, so that the temperature of the gas flow leaving the power is maintained above the dew point; practically all the liquid in the tower is evaporated and the reaction products are collected at the outlet of the tower in the solid state. A required quantity of a neutralizing agent is naturally proportional to the concentration of noxious components in the waste gases being processed; this quantity corresponds, under favorable conditions, to, for example, stoichiometric ratio of 1.5.
The quantity of water to be used must be at least sufficient to maintain the neutralizing agent in the state of aqueous solution or suspension. The known process is limited to such instances, in which the precalculated temperature drop of the gas caused by the evaporated water is smaller than the difference between the initial temperature of the gases and their dew point. With vapor generation particularly, the temperature, at which waste gases enter the purification system is relatively low, taking into consideration thermal efficiency; such a temperature may be, for example, 130 degrees -140 degrees C. The temperature at the end of the gas purification stage must be at least between 100 degrees and 110 degrees C. in order to ensure a necessary safety range with respect to the dow point. The temperature drop in the gas purification is only about 30 degrees. This condition is possible with a coal and oil combustion for producing steam and with the use of a slacked lime as a neutralizing agent, and only when the sulfur content in the fuel is not substantially greater than 1%. Therefore, the so-called "wet-dry" process is not usable in the instances where a fuel with a high sulfur content, such as 2 to 3%, is utilized in burners.
German patent publication No. DE-OS 25 20 045 discloses a method, in which finely pulverized dry absorption masses, such as lime dust, are spray-injected into crude gases. The temperature reduction in this method is practically avoided. The disadvantage of this, otherwise satisfactory method is that the amount of noxious gases cannot be reduced sufficiently unless the neutralizing agent is supplied into the reaction container in such a quantity that the stoichiometric ratio is multiply exceeded. This is very costly and in many cases non-attainable.
The German published application No. 25 18 079 discloses a two-stage process, in which the first stage is "wet-dry" and the second stage following the first stage is dry. A larger portion of the noxious gases, for example, about 90% is removed from the waste gases in the first "wet-dry" stage. Only relatively small remaining quantity of noxious gases is withdrawn from the gases being processed in the dry stage. It should be noted that the above described method is not economical in use of the dry stage with gases having a high concentration of noxious substances because only 7 to 15% of the dry agent are efficient for an absorption of the noxious components. Since nearly all the noxious components are separated from the waste gases in the "wet-dry stage", the quantity of water required for the two-stage process is insignificantly smaller than that in the one-stage "wet-dry" method. There, again high concentrations of noxious gases require high consumption of water, which causes a strong temperature drop as has been mentioned above. The example described in this German published application No. DE-OS 25 18 079 confirms that the temperature drop in the first stage amounts to several hundred degrees on the Celsius Scale. The conventional method, therefore, has the same limits in its use as the above discussed "wet-dry" methods.
A further known method for the absorption removal of pollutants from waste gases is known from the applicant's U.S. Pat. No. 4,208,383.